The present invention relates generally to hydraulically powered differential steered agricultural machines, and more particularly to an improved steering system for such machines.
Hydrostatic drive systems are often configured with the drive pump attached directly to the engine without a clutch or other mechanism to stop output flow of the pump while the engine is running. Drive pump displacement (output) and therefore speed in differential-steered machines is controlled by variations in swashplate angle which is adjusted by movement of external pintel arms external to the pumps. On most pumps, pintel arm and thus swashplate rotation from “neutral” to “full speed” occurs over a relatively narrow range on the order of 16-18 degrees. With such a small range of motion, it is important that the assembly controlling the angular position of the pintel arm angles be capable of precise angular control. Precise control of the swashplate angle for hydrostatic ground drive pumps on differential-steered agricultural machines is paramount, especially when operating the machine at higher travel speed such as is necessary for road transport between fields.
Conventional mechanical mechanisms use a threaded interface between a rotating shaft controlled by a steering wheel and the pintels arms on the drive pumps. Steering rate (generally, the number of steering wheel turns from full right lock to full left lock) is established by the thread pitch on this control interface and is selected to balance steering precision with an ergonomically acceptable steering rate. As thread pitch is not readily adjustable, the steering rate is a compromise between low-speed maneuverability, such as during harvesting operations, and high speed stability, such as during over-the-road transport.